1. Field of the Invention
The present invention is directed to a relay driving circuit, and more particularly to such a driving circuit for driving a magnetic relay of latch-in type to selectively set and reset the relay contact by charging and discharging a current to and from a capacitor connected in series with an excitation coil of the relay.
2. Description of the Prior Art
For driving a magnetic relay it is known in the art to provide a circuit in which a capacitor is connected in series with an excitation coil of the relay so that the relay can be set and reset into the contact closing and opening positions upon energization of the excitation coil selectively by charge and discharge currents of opposite polarity directed to and from the capacitor. FIG. 8 illustrates a general diagram of the known relay driving circuit which comprises a capacitor C connected in series with an excitation coil L of a magnetic relay, an input voltage level detector 10A connected to detect a level of voltage applied to the circuit, a set switch 20A connected in series with the series combination of the excitation coil L and the capacitor C, a reset switch 30A connected in parallel with the series combination of the coil L and the capacitor C. The input voltage level detector 10A compares the input voltage level with a predetermined trigger voltage level and produces a first control output when the input voltage level exceeds the trigger level and otherwise produces a second control output. In response to the first control signal the set switch 20A is rendered to be conductive while the reset switch 30A is kept non-conductive to thereby apply the input voltage to the series combination of the excitation coil L and the capacitor C for flowing a charge current through the excitation coil L in one direction, actuating the relay into a set position of closing the relay contact. At this time the capacitor C is charged for ready to discharge sufficient current through the excitation coil L in the opposite direction. In response to the second control signal from the input voltage level detector 10A, or when the input voltage is decreased below the trigger level, the reset switch 30A is made conductive to thereby establish a closed loop of the excitation coil L, the capacitor C, and the reset switch 30A, allowing the discharge current from the capacitor C to flow through the excitation coil L in the opposite direction, thus actuating the relay into a reset position of closing the relay contact. In this manner, the relay is set and reset by changing the level of the input voltage to the driving circuit.
The above described relay driving circuit is realized in the prior art, for example, by the circuit of FIG. 9. In the circuit, the input voltage level detector 10A comprises an operational amplifier OP which compares an input voltage divided by a divider network of resistors R.sub.1 and R.sub.2 with a reference level vref from a reference voltage source E.sub.1 to provide a high level output when the former is greater than the latter as representative of that the input voltage level exceeds a trigger voltage level. Otherwise, the operational amplifier OP.sub.1 produce a low level output as the second control signal. The set switch 20A comprises a pair of coupled transistors Q4 and Q5, the latter of which is inserted in series with the series combination of the excitation coil L. The reset switch 30A comprises a set of transistors Q6, Q8, and FET Q7, the last of which is connected across the series combination of the excitation coil L and the capacitor C. The transistor Q6 and FET Q7 are connected to derive its source of voltage from the capacitor C.
In operation, when the input voltage Vi is increased to such an extent that the divided voltage V.sub.1 becomes greater than the reference level Vref, the input voltage level detector 10A provides H-level output to turn on the transistors Q4 and Q5, whereby the input voltage Vi is applied to the series circuit of the excitation coil L, the capacitor C, and the transistor Q5 to charge the capacitor C with a current flowing through the excitation coil L in one direction. Thus, the relay is energized to one polarity and actuated into the set position. At this time, the transistor Q6 is kept turned on by the H-level output from the input voltage level detector 10A to thereby turn off the FET Q7 and the transistor Q8, rendering the reset switch 30A non-conductive. When the input voltage Vi is removed or decreased to an extent that the divided voltage V.sub.1 falls below the reference voltage Vref, the detector 10A provides a low-level output to thereby turn off the transistors Q4 and Q5, making the set switch 20A non-conductive and therefore disallowing the current to flow in the same direction through the excitation coil L. At this time, the transistor Q6 is turned off in response to the L-level output from the detector 10A to thereby turn on the FET Q7 and the transistor Q8 to establish the closed loop of the excitation coil L, the capacitor C and the transistor Q8. Whereby the capacitor C is allowed to discharge a current of the opposite direction through the excitation coil L for actuating the relay into the reset position of closing the relay contact.
However, the above circuit of FIG. 9 is found to have a serious problem in that there may be an unacceptable delay in actuating the relay into the reset position from the set position. Such delay comes from the fact that even after the input voltage is decreased below the trigger level in order to reset the relay, the input voltage detector 10A Will receive the voltage developed across the capacitor C to continuously provide the H-level output, thereby keeping the transistor Q5 turned on while keeping the transistor Q8 still turned off and therefore disallowing the capacitor C to discharge the reset current through the excitation coil L. This is true as the transistor Q5 will act to reversely flow a current (as indicated by an arrow in the figure) from the capacitor C through the excitation coil L when the input voltage is decreased to zero or below the critical level. Consequently, the input voltage level detector 10A responds in an unintended manner to still provide the H-level output until the capacitor C is discharged to a certain extent, thus causing the delay in turning on the transistor Q8 and resetting the relay.
To eliminate the above delay or the unintended reverse current flow from the capacitor to the detector 10A, there has been proposed an improved relay driving circuit. In the improved circuit, which is illustrated in FIG. 10, the transistors Q4 and Q5 forming the set switch 20B are connected in Darlington pair. With the Darlington connection, the transistor Q4 may flow a reverse current but the transistor Q5 will not allow the reverse current therethrough, inhibiting the unintended reverse current from the capacitor C to the detector 10B and therefore preventing the unintended operation of providing the H-level output from the detector 10B at the very moment of the input voltage decreasing to zero or below the trigger level.
Although the fault operation of the circuit, another preventing the faulty operation of the circuit, another problem has been encountered in using the Darlington circuit. That is, since the Darlington circuit requires a higher input voltage than a single transistor circuit for producing the set and reset currents of a prescribed level sufficient to magnetize the excitation coil, the circuit of FIG. 10 correspondingly requires more input power and is found to be unsatisfactory from the viewpoint of reducing the energy consumption. This is especially true when the relay driving circuit is adapted to a battery powered portable device in which energy saving is a primary concern.